1. Field of the Invention
The present invention is directed to packet-based communications systems, and more particularly, to a method for providing packet error indication in a multi-hop communications system.
2. Description of the Prior Art
In a packet based multi-hop communications system, data is transmitted in packets from a transmission site to a reception site via a plurality of successive physical links. For example, in future digital subscription television systems, packetized video and audio data may be transmitted from a programmer site to a cable head-end installation via a satellite transmission system, and after reception at the head-end, may be transmitted to individual subscriber locations via a cable distribution network. The satellite transmission system and cable distribution network each define a separate "physical link".
Typically, prior to transmission over a given physical link, the packets of data to be transmitted are encapsulated in a "data link layer" (as defined in the International Standards Organization (ISO) Open System Interconnection (OSI) reference model (ISO 7498)) that provides, among other things, forward error correction capabilities. Essentially, a forward error correcting code is appended to each packet, or a series of packets, prior to launching the packets over the physical link. At the terminal side of the physical link, the error correcting code is employed to correct, if possible, any errors that may have occurred during transmission over that physical link. Popular error correcting techniques include Reed-Solomon block error coding and convolutional coding.
Although error correcting codes are capable of correcting certain data errors, some errors may be so extensive that correction is impossible. However, the error correcting techniques are at least able to identify such uncorrectable errors. In prior art multi-hop communications systems, when an uncorrectable packet error is detected by the receiving equipment of a particular physical link, the erroneous packet is simply discarded and the communications system must deal with the loss of that packet; most systems assume that the data in a corrupted packet is useless. However, certain kinds of data, such as coded video data, may be useful despite an uncorrectable data error, and it would therefore be desirable to allow packets having uncorrectable data errors to continue through successive physical links to the ultimate reception site. However, merely allowing a corrupted packet to propagate through the system presents a problem, however, in that there is no way of alerting the ultimate reception site that an error occurred during transmission of that packet over a one or more of the physical links. A need arises therefore for a method of providing an indication to a reception site in a multi-hop communications system that an uncorrectable error occurred during transmission of a packet over one of the physical links in the communications system. The present invention satisfies this need.